In recent years, III nitride semiconductors generally made of compounds in which N is combined with Al, Ga, In, and the like are widely used for light emitting devices, elements for electronic devices, and the like. The characteristics of such devices greatly depend on the crystallinity of III nitride semiconductors; therefore, techniques for growing highly crystalline III nitride semiconductors are demanded.
A III nitride semiconductor is formed by performing epitaxial growth on a substrate made of sapphire, SiC, Si, GaAs, or the like. However, the lattice constant and the coefficient of thermal expansion are significantly different between the III nitride semiconductor and such a substrate. Accordingly, there has been a problem in that, if the III nitride semiconductor is grown on the substrate, the grown-up III nitride semiconductor causes cracks and pits (point-like defects). In view of this, it is known that a highly crystalline III nitride layer is grown by forming a buffer layer on a substrate, and then epitaxially growing a III nitride semiconductor layer on the buffer layer, thereby preventing the formation of cracks and pits due to the strain buffer effect of the buffer layer.
PTL 1 (JP 2007-067077A) discloses a technique of producing a highly crystalline III nitride semiconductor layer, in which crack formation is prevented, on a Si substrate by providing an AlN-based superlattice buffer layer, in which a plurality of first layers made of AlxGa1-xN (Al content x: 0.5≦x≦1) and a plurality of second layers made of AlyGa1-yN (Al content y: 0.01≦y≦0.2) are alternately stacked, between the Si substrate and the III nitride semiconductor layer.
PTL 2 (JP 2009-158804A) discloses a technique of obtaining a highly crystalline III nitride semiconductor layer. The III nitride semiconductor layer is formed on the superlattice composite layer by forming an AlN buffer layer on a Si substrate and sequentially stacking, on the AlN buffer, a composition graded layer having a composition graded such that the Al content decreases in the crystal growth direction, and a superlattice composite layer, in which high Al-content layers and low Al-content layers are alternately stacked.